1. Field of the Invention
The present invention relates to image forming apparatus.
2. Description of the Related Art
Image forming apparatus, such as electro-photographic printers, copiers, and facsimiles, are well known. An image forming apparatus has a medium transporting unit and a plurality of image forming units. The medium transporting unit transports a medium to the image forming units, which are disposed along the medium transport passage. Each image forming unit has a printing process cartridge, an exposing section, and a transfer section to form a toner image. The transfer section transfers to a medium the toner image that is made in the printing process cartridge. The printing process cartridge is detachable from the image forming unit. It has a charging section, a photosensitive body, a developing section, a cleaning section, a toner cartridge, and a drive transmitting section.
The drive transmitting section transmits power for driving the charging, photosensitive, and developing sections. The charging section charges the photosensitive body. The exposing section illuminates the surface of the charged photosensitive body to form an electrostatic latent image on the photosensitive surface according to the printing data. The developing section adheres a toner to the latent image on the photosensitive body to make a toner image that is visible. The cleaning section scrapes off the remaining toner from the photosensitive body. The toner cartridge, which is detachable from the printing process cartridge, supplies the developing section with toner.
The developing section is provided with a developing roller and a toner supply roller. The developing roller is pressed against the surface of the photosensitive body to adhere toner to the latent image on the photosensitive body. The transfer section transfers the toner image onto a recording medium from the photosensitive body. The fixing section has a heat roller and a heater for heating the heat roller to fix the toner image on the recording medium. Also, it has a pressure roller for pressing the heat roller against the recording medium. The heater of the fixing section produces heat at the time of printing and the recording medium and the toner thereon absorb the heat, keeping the image forming unit at a relatively low temperature.
In order to speed up the image forming process, the heater is made to produce heat at a time when no printing is made, when the recording medium and the toner thereon do not absorb the heat. Consequently, a large amount of heat is conducted from the fixing section to the image forming unit, raising its temperature very high. As a result, the toner is melted and deformed to adhere to the image forming unit. The thermally deformed toner degrades the quality of an image formed on the recording medium.
Thus, the temperature of the fixing heater is lowered when no printing is made so that a small amount of heat is conducted to heat the image forming unit, keeping its temperature at a low level. Consequently, the toner is kept normal in the image forming unit. However, the subsequent image forming process takes a long time because it takes a long time to bring the fixing unit from the low temperature to the high temperature, reducing the image forming speed and through-put.